A dual active bridge (DAB) DC-DC converter is ideally suited for high-power, galvanically isolated DC-DC conversion. The DAB DC-DC converter has advantages of high power density, Zero Voltage Switching (ZVS), bidirectional power transfer capability, a modular and symmetric structure, and simple control requirements. The DAB DC-DC converter can also be used for multi-port operation, which is a feature that is useful in interfacing several DC sources and loads using a single converter. Although several other bidirectional isolated DC-DC converter topologies have been proposed in literature, the simple symmetric structure and simple control mechanism of the DAB DC-DC converter are unique attributes. The DAB DC-DC converter has also been proposed as a building block for modular high power converters.
FIG. 1A is a schematic of a conventional DAB DC-DC converter 10 (hereinafter “DAB converter 10”). FIG. 1B illustrates the operating waveforms of the DAB converter 10 of FIG. 1A. As illustrated in FIG. 1A, the DAB converter 10 includes two H-bridges 12 and 14 (labeled HB1 and HB2, respectively) formed by switches S1-S4 and S1s-S4s connected as shown. The DAB converter 10 also includes a transformer 16 constructed with a relatively high and controlled leakage inductance (L) represented as inductor 18. Since a high inductance (L) is required, either an external inductance or an integrated magnetic structure incorporating a series inductance, as shown in FIG. 1A, is used. The H-bridges 12 and 14 are operated at 50% duty ratio with their outputs phase-shifted by a controlled angle, or phase shift, φ. The difference between voltages vp and vs of the H-bridges 12 and 14, respectively, appears across the inductor 18 representing the leakage inductance (L) of the transformer 16 and determines a transformer current (IL) of the transformer 16. The normalized power transfer provided by the DAB converter 10 is given by:P=mφ(1−|φ|/π); Pbase=Vdc12/XL  Eqn. (1)where XL=2πfswL, fsw is the switching frequency, L is the leakage inductance, m=NpsVds2/Vdc1, Nps is the primary to secondary turns ratio (Np/Ns), Vdc1 is the voltage across the H-bridge 12, and Vdc2 is the voltage across the H-bridge 14. Both the magnitude and direction of power transfer are controlled by the phase shift φ. For φ>0, as in FIG. 1B, the H-bridge 12 leads the H-bridge 14, and power is transferred from Vdc1 to Vdc2. For φ<0, power is transferred in the reverse direction. Due to the leakage inductance (L), the current output of each of the H-bridges 12 and 14 lags the voltage output and discharges the switch capacitances during the dead times thereby achieving ZVS. However, the load range over which ZVS of all switches is achieved (i.e., the ZVS range of the DAB converter 10) is limited particularly when the input or output voltages vary significantly.
Notwithstanding all of the advantages of the conventional DAB converter 10, for applications requiring wide voltage variations, such as an interface for energy storage, fuel cells, or photovoltaics, the DAB converter 10 has limited ZVS range and high circulating currents at low loads. The high circulating currents at low loads results in poor efficiency when the DAB converter 10 is under a low load condition. Thus, there is a need for an improved DAB converter that provides an increased ZVS range and/or increased efficiency particularly at low load conditions.